ZOOLOaT AND BOTANY, MICROSCOPY, ETC. 227 



It appears that the shell-gland is not formed by a mere thickening 

 of the epiblast, but is an actual invagination as in other lamellibranchs 

 and gasteropods ; subsequently it becomes a simple thickening of 

 epiblast and secretes a delicate chitinous membrane — the first trace of 

 a shell ; the two valves are not formed independently as was stated 

 by Lacaze-Duthiers ; thus the shell appears to originate in all Mollusca 

 after the same fashion, and accordingly v. Ihering's idea that the 

 Mollusca are diphyKc must be received with great distrust. The 

 embryo next acquires a preoral circlet of cilia in the middle of which 

 is a thickening of epiblast, the cephalic disk, which gives rise to 

 the cerebral ganglia. In older larvte the adductor muscle occupies a 

 place which shows it to correspond to the anterior adductor of the 

 Dimyaria, while in the adult the single adductor is the homologue of 

 the posterior adductor. The branch ite are developed as single filaments 

 united only at their bases — an argument in favour of regarding 

 filamentous branchite as a more ancient condition than the lamellar 

 branchias of adult bivalves. Dr. Hoek made a number of very careful 

 experiments (by means of an apparatus figured in the text) to determine 

 how the young fix themselves ; his results are not positive but he 

 inclines to the belief that there is a small byssus present. 



Byssogenous Glands and Aquiferous Pores in Lamellibranchs.* 

 — T. Barrois points out that, in the present state of our knowledge, 

 it is generally admitted that the byssus is the secretion of special 

 glands (glandula byssipara) ; he is now able to demonstrate that, 

 with rare exceptions, all lamellibranchs exhibit more or less well- 

 marked traces of this byssogenous apparatus. 



Taking Cardium edule as our type, we there find the organ con- 

 sisting of a groove, placed on the lower surface of the foot, of glands 

 at the sides of the groove, of a canal which extends from it to the 

 more or less spacious cavity of the byssus, and of compact masses of 

 byssogenous glands which pass the products of their secretion into 

 the cavity. This last, as well as the canal and the groove, is lined 

 by a cylindrical epithelium. In C edule the byssus is formed by a 

 simple hyaline filament. 



In more developed types (Mytilus, Pinna, &c.) the glands are much 

 denser, and the anterior extremity of the foot is prolonged to form 

 the linguiform muscle ; the lamellae of the cavity of the byssus are 

 more numerous, and the filaments form a tuft. In Anomia differen- 

 tiation is carried much further, for the byssus becomes charged with 

 carbonate of lime, and forms the ossicle. 



In the types where the organ undergoes retrogression, we find 

 that there may be a very short groove, or none, and no glands {T)onax, 

 Tellina) ; sometimes the groove and cavity are present, but no glands 

 (Nucula) ; sometimes only a delicate blind canal can be distinguished, 

 formed by a simple layer of epithelial cells (Psammobia tellina). In 

 Pholas or Solen there are no signs of groove, cavity, or glands. 



As the author has examined more than fifty species and types of. 

 every family except the Trigoniidse and Tridacnidse, and has found 



* Comptes Eeudus, c. (1885) pp. 188-90. 



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